1. Field of the Invention
This invention relates to the so-called denitration catalyst for reducing nitrogen oxides contained in exhaust gases with ammonia to remove the oxides from the gases. More particularly it relates to a catalyst for removing nitrogen oxides, which is suitable for treating exhaust gases containing a large amount of volatile metal oxides.
2. Description of the Related Art
Nitrogen oxides (NO.sub.x) exhausted from fixed generating sources such as various combustion equipments including boilers have formed a cause of air pollution together with sulfur oxides (SO.sub.x). Various processes for removing NO.sub.x have been proposed, and among these, the so-called catalytic ammonia reduction denitration process of adding ammonia (NH.sub.3) into exhaust gases, followed by contacting the mixture with a catalyst to reduce NO.sub.x into nitrogen has constituted a main current. As to the catalyst used in the above-mentioned process, it is necessary that the catalyst be not deteriorated by SO.sub.x contained in a large amount in the combustion gas of fuels such as petroleum, coal, etc. and also by ashes in the fuels. As catalysts satisfying these requirements, various catalysts based on titanium oxide have been developed and broadly used (see Japanese patent application laidopen Nos. Sho 50-128681/1975 and Sho 53-281485/1978). These catalysts have been obtained by adding compounds of transition metal elements such as vanadium (V), molybdenum (Mo), tungsten (W), iron (Fe), chromium (Cr), etc. to metatitanic acid or titanium oxide, followed by calcining the mixture. They have a superior activity and life as a catalyst for treating combustion exhaust gases from usual petroleum or coal.
However, in the case of combustion exhaust gases of low quality coal or in the case of boilers provided with an ash-circulating line very often employed in Europe, a large amount of volatile oxide vapor of heavy metals, etc. is contained in exhaust gases and when such exhaust gases are treated, the catalyst activity is rapidly deteriorated. FIG. 2 shows a flow diagram of a conventional boiler system having an ash-circulating line. Ashes collected at electrostatic precipitator 7 are recycled through ash-circulating line 9 to boiler furnace 1. Numeral 10 shows denitration apparatus. Numerals 2, 6 and 8 show economizer, air-preheater and chimney, respectively. In the case of the combustion system described above provided with such an ash-circulating line 9, there has been raised such a problem that elements contained in the mineral substances in coal such as lead (Pb), selenium (Se), arsenic (As), cadmium (Cd), zinc (Zn), etc. are transferred into exhaust gases in the form of solid substances or vapors during the process wherein ashes are melted inside a boiler furnace 1 and recovered as slag, and are present in the form of metal vapor in a high concentration upstream of an air preheater 6 provided with a denitration apparatus 10 (H. Brunisark et al., Environmental Technology, Letters; 5, 7-22 (1987)) so that the nitration catalyst is poisoned by such vapor.